Most tires are filled with compressed air, which when dry consists of about 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases by volume. Water vapor (humidity) can make up as much as 5 percent of the volume of air under worst-case conditions. Filling your tires with nitrogen mainly does two things: it eliminates moisture, and it replaces skinny oxygen molecules with fat nitrogen molecules, reducing the rate at which compressed gas diffuses through porous tire walls. That means, theoretically at least, that a tire filled with nitrogen retains optimal pressure longer, leading to more uniform tire wear and better gas mileage. The commonly quoted figure is that tires inflated to 32 psi get 3 percent better mileage than at 24 psi.

Does nitrogen make any practical difference? You couldn't prove it by me. I found no scientific tests showing that nitrogen-filled tires stayed inflated longer than average under normal conditions. A car-buff buddy was sure it worked but conceded he had only anecdotal evidence that it did.

Nitrogen is an inert gas that slows the formation of corrosion inside the wheels.
The bit about it being a fat molecule isnt really accurate, I ran a cryogenic gasseous spectra analyzer testing different types of gasses that we produced with cryogenic pumps (mostly Aviators Breathing Oxygen and Nitrogen) for the Navy at one of my duty stations for a few years and I can tell you from experience and the tech manuals that oxygen and nitrogen molecules are the same size (roughly since every molecule differs in size).
A benefit of nitrogen due to its low moisture and oxygen content is it isnt affected by temperatures and remains more stable than pressurized air when used in tires.
Aircraft have been using nitrogen vice air for years due to the rapid heating and cooling on tires during take-offs and landings.
I've yet to see any factual data that it increases fuel mileage though.

I've seen trade publications on this. The real benefit is to long haul truckers who re-tread their tires a few times. The lack of moisture is the key issue as it limits wheel corrosion and that helps keep the wheels inflated. Also truckers usually check tire pressure during regular PM inspections and use other technology to keep tire pressure where it belongs to limit heat induced 'zipper' failures caused by the imbalance of dual tires causing extra sidewall flex and fatigue of the tire with greater pressure.

Several studies have shown that it doesn't help gas mileage. Your best key to mileage is a tire pressure gauge used on a regular basis.

On the other hand; this is becoming the latest item displayed at the new car stealerships giving them another reason to reach deeper into the average person's pocket.

Here's some info from tirerack on nitrogen tire inflation.
Since it is available to me from costco at no cost I will stick with it and maybe I'll maintain better consistent tire inflation as the temperatures drop and less often having to fill them/top them off and less tire wear.
I have chrome wheels so the nitrogen may have some benefits against corrosion of the inner wheels. http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=191

On the other hand; this is becoming the latest item displayed at the new car stealerships giving them another reason to reach deeper into the average person's pocket.

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Yep, & if the governments "Cap & Trade" (Crap & Tax) passes, we will all be required to use nitrogen I'm sure. Not so bad for the anti-corrosion properties, but I can just see the lines of cars in small towns where there is only ONE SUPPLIER with one pump for miles around who is legally authorized by the government to pump up your tires for you..ha ha ha

I will continue to use compressed air. It's done its job for many years, with minimal if any increase in gas mileage and no real hard evidence that it is proven to be better. I won't switch to nitrogen, until my local gas station offers it and I can buy my own nitrogen filling compressor for my own garage. I have no need to spend any money to fill my tires with.

I will continue to use compressed air. It's done its job for many years, with minimal if any increase in gas mileage and no real hard evidence that it is proven to be better. I won't switch to nitrogen, until my local gas station offers it and I can buy my own nitrogen filling compressor for my own garage. I have no need to spend any money to fill my tires with.

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Ingersoll-Rand offers portable and stationary Nitrogen servicing systems for home use, a little expensive at this time.
I agree I've never had problems with regular old air in my tires and I'll continue to use it as long as I dont have to pay extra for it.
My wife has Nitrogen in her HHR tires and during the last service they offered to fill them to the proper pressures for $20, needless to say she brough =t the car home and had me service the tires with my air compressor.

Ingersoll-Rand offers portable and stationary Nitrogen servicing systems for home use, a little expensive at this time.
I agree I've never had problems with regular old air in my tires and I'll continue to use it as long as I dont have to pay extra for it.
My wife has Nitrogen in her HHR tires and during the last service they offered to fill them to the proper pressures for $20, needless to say she brough =t the car home and had me service the tires with my air compressor.

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Since 70% of that compressed air IS nitrogen, you won't be diluting it much...

My wife has Nitrogen in her HHR tires and during the last service they offered to fill them to the proper pressures for $20, needless to say she brough =t the car home and had me service the tires with my air compressor.

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LOL Tim, glad your wife is smarter than most. Not only would my past w/gfs have fallen for the new nitro b.s. & happily paid $40. for the fill, they'd have been so excited, they'd have been out there waving people in off the street to buy some ha ha ha:rofl: